Not Applicable
This invention relates generally to the field of containment of underground hazardous wastes and more specifically to a method and apparatus for constructing an underground barrier wall structure using a jet grout injector subassembly comprising a pair of primary nozzles and a plurality of secondary nozzles, the secondary nozzles having a smaller diameter than the primary nozzles, for injecting grout in directions other than the primary direction, which creates a barrier wall panel having a substantially uniform wall thickess.
An important goal of environmental remediation is reducing or preventing underground hazardous wastes from migrating outside of contaminated sites. Examples of hazardous wastes include pesticide contaminated groundwater, benzene vapors, or non-aqueous phase liquids, such as gasoline leaking from a buried storage tank. An underground structure, such as a barrier wall, can be used to contain or redirect the flow of groundwater contaminated with hazardous wastes. A barrier wall is typically made of a substantially impermeable material that prevents the flow of these hazardous materials through the relatively permeable surrounding ground (soil, sand, etc.). Cement-based grout (a well-known mixture of Portland cement, sand, and water), sometimes mixed with the surrounding soil, is commonly used as a ground-hardening material to fabricate impermeable underground barrier walls.
Alternatively, the underground barrier wall can be constructed of materials that include permeable reactive materials (PRM""s). As the hazardous wastes flows through the permeable barrier wall, the wastes are removed, captured, or modified by the action of various active agents contained within the PRM""s. The PRM""s react with the hazardous wastes by chemical, physical, or biological processes, or combinations of these. Treated groundwater is then returned to the aquifer.
Underground barrier walls can be fabricated in-situ by jet grouting. The term xe2x80x9cjet groutingxe2x80x9d refers to the use of high-pressure jet spray nozzles, which are located on an injector subassembly attached to the end of a drill string, to inject a slurry of material at relatively high velocity into the surrounding soil. The jet spray simultaneously masticates and erodes the surrounding soil, while mixing the loosened soil with the injected slurry to form a soil/slurry mixture that replaces the eroded cavity. If the slurry is primarily made of grout then the mixture of soil and grout subsequently hardens into a solid, substantially impermeable material (sometimes called xe2x80x9csoilcretexe2x80x9d). If the slurry contains PRM""s, then the mixture of soil and slurry forms a permeable reactive barrier wall.
In this application, the term xe2x80x9csoilxe2x80x9d is broadly defined to include any mixture of soil, sand, clay, gravel, organic materials, or other granular materials, either naturally occurring or man-made, which can be loosened and eroded by the action of the jet spray. The phrase xe2x80x9cjet groutingxe2x80x9d is broadly defined to include injection of slurries containing (1) grout or other ground-hardening materials; or (2) permeable reactive materials (PRM""s). The terms xe2x80x9cslurryxe2x80x9d and xe2x80x9cgroutxe2x80x9d is herein broadly defined to include mixtures of solid materials with any liquid, including water; and with any gases, including air. The terms xe2x80x9cslurryxe2x80x9d and xe2x80x9cgroutxe2x80x9d also comprehends 0% of solid materials, including: (1) injection of only liquids; (2) liquids plus gases; (3) gases only; (4) or any combination of solids, gases, and liquid that can be injected from a spray nozzle, e.g. xe2x80x9cjet groutedxe2x80x9d. In this application, the terms xe2x80x9cslurryxe2x80x9d and xe2x80x9cgroutxe2x80x9d are used interchangeably, as defined herein above.
Jet grouting typically occurs when the drill string is being withdrawn from the drill hole. If the jet injector subassembly is not rotated during withdrawal, then the jet spray creates a thin xe2x80x9cdiaphragm wallxe2x80x9d. The injection of xe2x80x9cgroutxe2x80x9d as it was broadly defined earlier, from each jet nozzle, as the nozzle is removed from a single hole, acts to create its own thin diaphragm wall segment. The number of segments equals the number of jet nozzles. For example, operation of two jet nozzles would result in two panels. Each segment is connected to each other segment by grout which is deposited and fills up the central void space left when the drill string is removed from the drill hole.
The jet injector subassembly traditionally has only two nozzles (e.g. orifices) that typically that face outwards in diametrically opposite (e.g. 180 degrees opposed) directions. Nozzle diameters typically vary from 2 to 3 mm, but can be larger, or smaller. The slurry is injected at high pressures (up to 6000 psi) through these two nozzles, in a direction radially outward from the center of the subassembly. As illustrated in FIG. 1, the high velocity jet spray creates a panel whose width, Rmax, is greater than the panel""s maximum thickness, Tmax. Depending on the soil conditions, Rmax can be at least 2 meters; the maximum thickness Tmax can be at least 20-30 cm; and the minimum thickness Tmin can be at least 8-10 cm.
An interconnected barrier wall can be made by drilling a second hole close to the first one and repeating the jet grouting process, as many times as necessary to provide adequate coverage. FIG. 2 illustrates a series of interconnected thin diaphragm wall panels using this technique. Typical distances between adjacent holes are 1-3 meters, but can be larger or smaller depending on the soil conditions and requirements.
Conventional jet grouting processes that use only two (non-rotating) nozzles create barrier walls that have a non-uniform wall thickness. This results because the natural shape formed by a jet spray is an expanding cone. Consequently, when two nozzles inject slurry from diametrically opposed positions, a xe2x80x9cbow-tiexe2x80x9d shape results. The xe2x80x9cbow-tiexe2x80x9d shape can be seen in FIGS. 1 and 2.
The problem with this xe2x80x9cbow-tiexe2x80x9d shape is the thin, weak region located directly adjacent to the drill hole. This thin region is more prone to cracking, separating, and/or tearing than the thicker region at the end of the jet spray. Cracking may be caused by non-uniform shrinkage of the solidifying grout or surrounding media. Also, the thin region may have a non-optimum mixture of grout plus soil, when compared to the thicker region at the end of the jet spray. FIG. 3 shows that cracking of the thin section can create uncontrolled leakage of hazardous wastes, thereby defeating the integrity of the containment barrier.
The problem of weakness associated with the bow-tie shape is also present in permeable reactive barrier walls constructed of permeable reactive materials (PRM""s). Any large differences in the wall thickness of a porous reactive barrier wall (Tmax greater than  greater than Tmin) would likely result in non-uniform rates of waste treatment, and non-optimum utilization of the PRM""s. Likewise, cracking or tearing of the porous reactive barrier wall would reduce the overall effectiveness of the waste treatment process because untreated wastes could flow directly through the cracked region.
A need remains, therefore, for a simple and easily deployable solution to the problem of weakness and non-uniform thickness caused by the bow-tie shape associated with jet grout injection using conventional dual-nozzle technology. Against the background just described, the present invention solves this problem by using at least four additional secondary nozzles to simultaneously fill in the thin, weak region by injecting slurry in directions other than the pair of diametrically-opposed primary nozzles. FIG. 4 shows that this method produces a filled-in zone, 72, that creates an optimum underground barrier wall structure having a substantially uniform wall thickness. Throughout this application, the phrase xe2x80x9csubstantially uniformxe2x80x9d means that the variations in the wall""s actual thickness are small when compared to the wall""s average thickness.
The present invention is a method and apparatus for constructing underground barrier walls having a substantially uniform wall thickness. The apparatus has at least four secondary nozzles, of a smaller diameter than the primary nozzles, located on either side of the two primary nozzles. Slurry is injected simultaneously through the secondary nozzles in directions other than the primary direction in such a way that the thin regions of the bow-tie shape are filled in. The number, size, and location of the secondary nozzles are optimized depending on the soil and slurry properties. The secondary nozzles have a smaller diameter so that the flow and velocity of injected slurry is less than the spray of slurry from the primary nozzles. The lower velocity from the secondary nozzles reduces the depth of penetration. The smaller depth of penetration permits the thin region to be filled-in with a minimum additional amount of slurry, thereby maximizing efficiency. The result, shown in FIG. 4, is a panel having a substantially uniform wall thickness.
The injected slurry may include mixtures of solids, liquids, and gases, depending on the specific desired effect. Examples of ground-hardening that subsequently harden into a substantially impermeable barrier include: grout (a mixture of cement, sand, and water); and mixtures of grout with soil, sand, gravel, bentonite clay, fly ash, ground granulated blast furnace slag, or natural clay. Chemical additives can be added to either accelerate or slow down the hardening process. Air, or other gases, can be added to the mixture to modify the performance under different environments, such as freezing temperatures.
Many different methods can be used inside a porous reactive barrier wall to treat hazardous wastes, including: (1) Chemical Precipitation; (2) Oxidation-Reduction Reactions; (3) Zero-Valent Metal Dehalogenation (e.g. granular iron); (4) Biological Degradation Reactions; (5) Sorption of Organics; and (6) Sorption of Inorganics. Some examples of permeable reactive materials include activated carbon, zeolites, and granular iron.
The jet injector subassembly is attached to the end of a drill string and lowered into a hole in the ground. The injected slurry is supplied to the subassembly at a high pressure (up to about 6000 psi). After insertion, the drill string is slowly withdrawn, without rotation, from the hole while slurry is discharged simultaneously from both primary and secondary nozzles. The jet sprays simultaneously masticate and erode the surrounding soil and mixes the injected slurry with the eroded soil, whereby a thin diaphragm wall of substantially uniform wall thickness is formed. Next, the drill string and injector subassembly is removed, repositioned, and reinserted into an adjacent hole, whereupon the entire process is repeated.
The location of the adjacent hole is chosen advantageously so that the edges of the panels touch each other to form an interconnected underground barrier wall system. The planar orientation of each adjacent thin diaphragm wall panel can either be oriented substantially parallel to, and in-line with, the adjacent panel; or alternatively angled back-and-forth to form a folded, accordion-like wall structure, while continuing to touch the edges of each panel with each other.
A substantially uniformly thick wall thereby prevents the problems of cracking associated with the thin region of the bow-tie shape associated with conventional dual-nozzle techniques.